Integrated piezoresistive and piezoelectric fusion force sensor

ABSTRACT

Described herein is a ruggedized microelectromechanical (“MEMS”) force sensor including both piezoresistive and piezoelectric sensing elements and integrated with complementary metal-oxide-semiconductor (“CMOS”) circuitry on the same chip. The sensor employs piezoresistive strain gauges for static force and piezoelectric strain gauges for dynamic changes in force. Both piezoresistive and piezoelectric sensing elements are electrically connected to integrated circuits provided on the same substrate as the sensing elements. The integrated circuits can be configured to amplify, digitize, calibrate, store, and/or communicate force values electrical terminals to external circuitry.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/485,026, filed on Aug. 9, 2019, which is a 35 USC 371 national phaseapplication of PCT/US2018/017572 filed on Feb. 9, 2018, which claims thebenefit of U.S. provisional patent application No. 62/456,699, filed onFeb. 9, 2017, and entitled “INTEGRATED DIGITAL FORCE SENSOR,” and U.S.provisional patent application No. 62/462,559, filed on Feb. 23, 2017,and entitled “INTEGRATED PIEZORESISTIVE AND PIEZOELECTRIC FUSION FORCESENSOR,” the disclosures of which are expressly incorporated herein byreference in their entireties.

FIELD OF TECHNOLOGY

The present disclosure relates to microelectromechanical (“MEMS”) forcesensing with piezoresistive and piezoelectric sensor integrated withcomplementary metal-oxide-semiconductor (“CMOS”) circuitry.

BACKGROUND

Force sensing touch panels are realized with force sensors underneaththe display area with certain sensor array arrangements. These touchpanels require the force sensors to provide high quality signals,meaning high sensitivity is essential. Existing MEMS piezoresistivesensors are suitable for such applications and are typically paired withextremely low noise amplifiers due to the low sensitivity of thesensors. Such amplifiers are expensive and tend to consume a lot ofpower. Piezoelectric sensors are highly sensitive in force sensingapplications, but only for dynamic changes in force (i.e., not staticforces). Therefore, piezoelectric sensors cannot provide accurate offsetinformation.

Accordingly, there is a need in the pertinent art for a low power, highsensitivity force sensor capable of sensing both static and dynamicforce with high sensitivity and accuracy.

SUMMARY

A MEMS force sensor including both piezoresistive and piezoelectricsensing elements on the same chip is described herein. The force sensorcan also include integrated circuits (e.g., digital circuitry) on thesame chip. In one implementation, the force sensor is configured in achip scale package (“CSP”) format. A plurality of piezoresistive sensingelements are implemented on the silicon substrate of the integratedcircuit chip. In addition, a plurality of piezoelectric elements aredisposed between the metal pads and solder bumps, where the force isdirectly transduced for sensing.

The MEMS force sensor can be manufactured by first diffusing orimplanting the piezoresistive sensing elements on a silicon substrate.Then, the standard integrated circuit process (e.g., CMOS process) canfollow to provide digital circuitry on the same silicon substrate. Theoverall thermal budget can be considered such that the piezoresistivesensing elements can maintain their required doping profile. After theintegrated circuit process is completed, the piezoelectric layer alongwith two electrode layers (e.g., a piezoelectric sensing element) arethen disposed and patterned on the silicon substrate. Solder bumps arethen formed on the metal pads and the wafer is diced to create a chipscale packaged device. The force exerted on the back side of the deviceinduces strain in both the plurality of piezoresistive sensing elementsand the plurality of piezoelectric sensing elements, which producerespective output signals proportional to the force. The output signalscan be digitized by the integrated circuitry and stored in on-chipbuffers until requested by a host device.

An example microelectromechanical (“MEMS”) force sensor is describedherein. The MEMS force sensor can include a sensor die configured toreceive an applied force. The sensor die has a top surface and a bottomsurface opposite thereto. The MEMS force sensor can also include apiezoresistive sensing element, a piezoelectric sensing element, anddigital circuitry arranged on the bottom surface of the sensor die. Thepiezoresistive sensing element is configured to convert a strain to afirst analog electrical signal that is proportional to the strain. Thepiezoelectric sensing element is configured to convert a change instrain to a second analog electrical signal that is proportional to thechange in strain. The digital circuitry is configured to convert thefirst and second analog electrical signals to respective digitalelectrical output signals.

Additionally, the piezoresistive sensing element can be formed bydiffusion or implantation. Alternatively, the piezoresistive sensingelement can be formed by polysilicon processes from an integratedcircuit process.

Alternatively or additionally, the MEMS force sensor can include asolder ball arranged on the bottom surface of the sensor die. Thepiezoelectric sensing element can be disposed between the solder balland the sensor die.

Alternatively or additionally, the MEMS force sensor can include aplurality of electrical terminals arranged on the bottom surface of thesensor die. The respective digital electrical output signals produced bythe digital circuitry can be routed to the electrical terminals. Theelectrical terminals can be solder bumps or copper pillars.

Alternatively or additionally, the digital circuitry can be furtherconfigured to use the second analog electrical signal produced by thepiezoelectric sensing element and the first analog electrical signalproduced by the piezoresistive sensing element in conjunction to improvesensitivity and accuracy. For example, the first analog electricalsignal produced by the piezoresistive sensing element can measure staticforce applied to the MEMS force sensor, and the second analog electricalsignal produced by the piezoelectric sensing element can measure dynamicforce applied to the MEMS force sensor.

Alternatively or additionally, the MEMS force sensor can include a capattached to the sensor die at a surface defined by an outer wall of thesensor die. A sealed cavity can be formed between the cap and the sensordie.

Alternatively or additionally, the sensor die can include a flexureformed therein. The flexure can convert the applied force into thestrain on the bottom surface of the sensor die.

Alternatively or additionally, a gap can be arranged between the sensordie and the cap. The gap can be configured to narrow with application ofthe applied force such that the flexure is unable to deform beyond itsbreaking point.

Alternatively or additionally, the MEMS force sensor can include aninter-metal dielectric layer arranged on the bottom surface of thesensor die. The piezoelectric sensing element can be arranged on theinter-metal dielectric layer.

Alternatively or additionally, the digital circuitry can be furtherconfigured to store the respective digital electrical output signals toa buffer.

Other systems, methods, features and/or advantages will be or may becomeapparent to one with skill in the art upon examination of the followingdrawings and detailed description. It is intended that all suchadditional systems, methods, features and/or advantages be includedwithin this description and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE FIGURES

The components in the drawings are not necessarily to scale relative toeach other. Like reference numerals designate corresponding partsthroughout the several views. These and other features of will becomemore apparent in the detailed description in which reference is made tothe appended drawings.

FIG. 1A is an isometric view of the top of an example MEMS force sensoraccording to implementations described herein.

FIG. 1B is an isometric view of the bottom of the MEMS force sensor ofFIG. 1.

FIG. 2 is a cross-sectional view of an integrated p-type MEMS-CMOS forcesensor using a piezoresistive sensing element (not to scale) accordingto implementations described herein.

FIG. 3 is a cross-sectional view of an integrated n-type MEMS-CMOS forcesensor using a piezoresistive sensing element (not to scale) accordingto implementations described herein.

FIG. 4 is a cross-sectional view of an integrated p-type MEMS-CMOS forcesensor using a polysilicon sensing element (not to scale) according toimplementations described herein.

FIG. 5 is an isometric view of the top of another example MEMS forcesensor according to implementations described herein.

DETAILED DESCRIPTION

The present disclosure can be understood more readily by reference tothe following detailed description, examples, drawings, and theirprevious and following description. However, before the present devices,systems, and/or methods are disclosed and described, it is to beunderstood that this disclosure is not limited to the specific devices,systems, and/or methods disclosed unless otherwise specified, and, assuch, can, of course, vary. It is also to be understood that theterminology used herein is for the purpose of describing particularaspects only and is not intended to be limiting.

The following description is provided as an enabling teaching. To thisend, those skilled in the relevant art will recognize and appreciatethat many changes can be made, while still obtaining beneficial results.It will also be apparent that some of the desired benefits can beobtained by selecting some of the features without utilizing otherfeatures. Accordingly, those who work in the art will recognize thatmany modifications and adaptations may be possible and can even bedesirable in certain circumstances, and are contemplated by thisdisclosure. Thus, the following description is provided as illustrativeof the principles and not in limitation thereof.

As used throughout, the singular forms “a,” “an” and “the” includeplural referents unless the context clearly dictates otherwise. Thus,for example, reference to “a force sensor” can include two or more suchforce sensors unless the context indicates otherwise.

The term “comprising” and variations thereof as used herein is usedsynonymously with the term “including” and variations thereof and areopen, non-limiting terms.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another aspect includes from the one particular value and/orto the other particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

As used herein, the terms “optional” or “optionally” mean that thesubsequently described event or circumstance may or may not occur, andthat the description includes instances where said event or circumstanceoccurs and instances where it does not.

A MEMS force sensor 100 for measuring a force applied to at least aportion thereof is described herein. In one aspect, as depicted in FIG.1A, the force sensor device 100 includes a substrate 101 and inter-metaldielectric layer (IMD) 102 fabricated on a surface (e.g., bottomsurface) of the substrate 101 to form integrated circuits. The substrate101 can optionally be made of silicon. Optionally, the substrate 101(and its components such as, for example, boss, mesa, outer wall,flexure(s), etc.) is a single continuous piece of material, i.e., thesubstrate is monolithic. It should be understood that this disclosurecontemplates that the substrate can be made from materials other thanthose provided as examples. In another aspect, as depicted in FIG. 1B,the MEMS force sensor 100 is formed into a chip scale package withsolder bumps 103 and a plurality of piezoresistive sensing elements 104.The solder bumps 103 and the piezoresistive sensing elements 104 can beformed on the same surface (e.g., bottom surface) of the substrate 101on which the IMD layer 102 is fabricated. The piezoresistive sensingelements 104 are configured to convert a strain to an analog electricalsignal (e.g., a first analog electrical signal) that is proportional tothe strain on the bottom surface of the substrate 101. Thepiezoresistive sensing elements 104 detect static forces applied to theMEMS force sensor 100. This disclosure contemplates that thepiezoresistive sensing elements 104 can be diffused, deposited, orimplanted on the bottom surface of substrate 101.

The piezoresistive sensing elements 104 can change resistance inresponse to deflection of a portion of the substrate 101. For example,as strain is induced in the bottom surface of the substrate 101proportional to the force applied to the MEMS force sensor 100, alocalized strain is produced on a piezoresistive sensing element suchthat the piezoresistive sensing element experiences compression ortension, depending on its specific orientation. As the piezoresistivesensing element compresses and tenses, its resistivity changes inopposite fashion. Accordingly, a Wheatstone bridge circuit including aplurality (e.g., four) piezoresistive sensing elements (e.g., two ofeach orientation relative to strain) becomes unbalanced and produces adifferential voltage (also sometimes referred to herein as the “firstanalog electrical signal”) across the positive signal terminal and thenegative signal terminal. This differential voltage is directlyproportional to the force applied to the MEMS force sensor 100. Asdescribed below, this differential voltage can be received at andprocessed by digital circuitry (e.g., as shown in FIGS. 2-5). Forexample, the digital circuitry can be configured to, among otherfunctions, convert the first analog electrical signal to a digitalelectrical output signal.

Example MEMS force sensors using piezoresistive sensing elements aredescribed in U.S. Pat. No. 9,487,388, issued Nov. 8, 2016 and entitled“Ruggedized MEMS Force Die;” U.S. Pat. No. 9,493,342, issued Nov. 15,2016 and entitled “Wafer Level MEMS Force Dies;” U.S. Patent ApplicationPublication No. 2016/0332866 to Brosh et al., filed Jan. 13, 2015 andentitled “Miniaturized and ruggedized wafer level mems force sensors;”and U.S. Patent Application Publication No. 2016/0363490 to Campbell etal., filed Jun. 10, 2016 and entitled “Ruggedized wafer level mems forcesensor with a tolerance trench,” the disclosures of which areincorporated by reference in their entireties.

In addition, the MEMS force sensor 100 includes a plurality ofpiezoelectric sensing elements 105. The piezoelectric sensing elements105 are located between the solder bumps 103 and the IMD 102. Forexample, a piezoelectric sensing element 105 can be formed on the IMDlayer 102, and the solder bump 103 can be formed over the piezoelectricsensing element 105. The arrangement of a piezoelectric sensing element105 and the IMD layer 102 is shown in FIGS. 2-4. Referring again toFIGS. 1A-1B, the piezoelectric sensing elements 105 are configured toconvert a change in strain to an analog electrical signal (e.g., asecond analog electrical signal) that is proportional to the changestrain on the bottom surface of the substrate 101. The piezoelectricsensing elements 105 sense dynamic forces applied to the MEMS forcesensor 100. The second analog electrical signal can be routed to digitalcircuitry (e.g., as shown in FIGS. 2-5) arranged on the bottom surfaceof the substrate 101. For example, the digital circuitry can beconfigured to, among other functions, convert the second analogelectrical signal to a digital electrical output signal. Accordingly,the digital circuitry can be configured to convert the first and secondanalog electrical signals to respective digital electrical outputsignals. Additionally, the digital circuitry can be configured to storethe respective digital electrical output signals in a buffer such as anon-chip buffer.

In one implementation, as depicted in FIG. 2, the cross section of aMEMS force sensor device is shown. The force sensor device of FIG. 2 isa MEMS force sensor using an integrated p-type MEMS-CMOS force sensorwith a piezoresistive sensing element. The p-type silicon substrate 201is a CMOS chip with both an n-type metal-oxide-semiconductor (nMOS)transistor 210 and a p-type metal-oxide-semiconductor (pMOS) transistor211 fabricated on it. The p-type silicon substrate 201 can be a singlecontinuous piece of material, i.e., the substrate can be monolithic. ThenMOS source/drain 208 and pMOS source/drain 209 are formed throughdiffusion or implantation. As shown in FIG. 2, the pMOS source/drain 209reside in an n-well region 205, which receives a voltage bias through ahighly-doped n-type implant 215. Further, a gate contact 207 (e.g., polysilicon gate) forms the channel required for each of the nMOS transistor210 and pMOS transistor 211. It should be understood, however, thatsimilar CMOS processes can be adapted to other starting materials, suchas an n-type silicon substrate. Additionally, although a siliconsubstrate is provided as an example, this disclosure contemplates thatthe substrate can be made from a material other than silicon. Thisdisclosure contemplates that the MEMS force sensor can include aplurality of nMOS and pMOS devices. The nMOS and pMOS devices can formvarious components of the digital circuitry (e.g., CMOS circuitry). Thedigital circuitry can optionally include other components, which are notdepicted in FIG. 2, including, but not limited to, bipolar transistors;metal-insulator-metal (“MIM”) and metal-oxide-semiconductor (“MOS”)capacitors; diffused, implanted, and polysilicon resistors; and/ordiodes. The digital circuitry can include, but is not limited to, one ormore of a differential amplifier or buffer, an analog-to-digitalconverter, a clock generator, non-volatile memory, and a communicationbus. For example, the digital circuitry can include an on-chip bufferfor storing the respective digital electrical output signals.

In addition to the nMOS and pMOS transistors 210 and 211 shown in FIG.2, a lightly doped n-type piezoresistive sensing element 204 and aheavily doped n-type contact region 203 are formed on the same p-typesilicon substrate 201. In other words, the piezoresistive sensingelement and digital circuitry can be disposed on the same monolithicsubstrate. Accordingly, the process used to form the piezoresistivesensing element can be compatible with the process used to form thedigital circuitry. The lightly doped n-type piezoresistive sensingelement 204 and heavily doped n-type contact region 203 can be formed byway of either diffusion, deposition, or implant patterned with alithographic exposure process. The MEMS force sensor can also include apiezoelectric sensing element 105, which can be disposed on the IMD 102layer and underneath the solder ball 103. The piezoelectric sensingelement 105 can be formed after completion of the integrated circuitprocess. Metal 212 and contact 213 layers can be provided to createelectrical connections between nMOS and pMOS transistors 210 and 211,piezoresistive sensing element 204, and piezoelectric sensing element105. Accordingly, the MEMS force sensor includes a piezoresistivesensing element, a piezoelectric sensing element, and digital circuitryall on the same chip.

In another implementation, as depicted in FIG. 3, the cross section of aMEMS force sensor device is shown. The force sensor device of FIG. 3 isa MEMS force sensor using an integrated n-type MEMS-CMOS force sensorwith a piezoresistive sensing element. The p-type silicon substrate 201is a CMOS chip with both nMOS transistor 210 and pMOS transistor 211fabricated on it. The p-type silicon substrate 201 can be a singlecontinuous piece of material, i.e., the substrate can be monolithic. ThenMOS source/drain 208 and pMOS source/drain 209 are formed throughdiffusion or implantation. As shown in FIG. 3, the pMOS source/drain 209reside in an n-well region 205, which receives a voltage bias through ahighly-doped n-type implant 215. Further, a gate contact 207 (e.g., polysilicon gate) forms the channel required for each of the nMOS transistor210 and pMOS transistor 211. It should be understood, however, thatsimilar CMOS processes can be adapted to other starting materials, suchas an n-type silicon substrate. Additionally, although a siliconsubstrate is provided as an example, this disclosure contemplates thatthe substrate can be made from a material other than silicon. Thisdisclosure contemplates that the MEMS force sensor can include aplurality of nMOS and pMOS devices. The nMOS and pMOS devices can formvarious components of the digital circuitry (e.g., CMOS circuitry). Thedigital circuitry can optionally include other components, which are notdepicted in FIG. 3, including, but not limited to, bipolar transistors;metal-insulator-metal (“MIM”) and metal-oxide-semiconductor (“MOS”)capacitors; diffused, implanted, and polysilicon resistors; and/ordiodes. The digital circuitry can include, but is not limited to, one ormore of a differential amplifier or buffer, an analog-to-digitalconverter, a clock generator, non-volatile memory, and a communicationbus. For example, the digital circuitry can include an on-chip bufferfor storing the respective digital electrical output signals.

In addition to the nMOS and pMOS transistors 210 and 211 shown in FIG.3, a lightly doped p-type piezoresistive sensing elements 304 and aheavily doped n-type contact region 303 are formed on the same p-typesilicon substrate 201 inside an n-well 314. In other words, thepiezoresistive sensing element and digital circuitry can be disposed onthe same monolithic substrate. Accordingly, the process used to form thepiezoresistive sensing element can be compatible with the process usedto form the digital circuitry. The n-well 314, lightly doped n-typepiezoresistive sensing element 304, and heavily doped n-type contactregion 303 can be formed by way of either diffusion, deposition, orimplant patterned with a lithographic exposure process. The MEMS forcesensor can also include a piezoelectric sensing element 105, which isdisposed on the IMD 102 layer and underneath the solder ball 103. Thepiezoelectric sensing element 105 can be formed after completion of theintegrated circuit process. Metal 212 and contact 213 layers can beprovided to create electrical connections between the nMOS and pMOStransistors 210 and 211, piezoresistive sensing element 304, andpiezoelectric sensing element 105. Accordingly, the MEMS force sensorincludes a piezoresistive sensing element, a piezoelectric sensingelement, and digital circuitry all on the same chip.

In yet another implementation, as depicted in FIG. 4, the cross sectionof a MEMS force sensor device is shown. The force sensor device of FIG.4 is an MEMS force sensor using an integrated p-type MEMS-CMOS forcesensor with a polysilicon sensing element. The p-type silicon substrate201 is a CMOS chip with both nMOS transistor 210 and pMOS transistor 211fabricated on it. The p-type silicon substrate 201 can be a singlecontinuous piece of material, i.e., the substrate can be monolithic. ThenMOS source/drain 208 and pMOS source/drain 209 are formed throughdiffusion or implantation. As shown in FIG. 4, the pMOS source/drain 209reside in an n-well region 205, which receives a voltage bias through ahighly-doped n-type implant 215. Further, a gate contact 207 (e.g., polysilicon gate) forms the channel required for each of the nMOS transistor210 and pMOS transistor 211. It should be understood, however, thatsimilar CMOS processes can be adapted to other starting materials, suchas an n-type silicon substrate. Additionally, although a siliconsubstrate is provided as an example, this disclosure contemplates thatthe substrate can be made from a material other than silicon. Thisdisclosure contemplates that the MEMS force sensor can include aplurality of nMOS and pMOS devices. The nMOS and pMOS devices can formvarious components of the digital circuitry (e.g., CMOS circuitry). Thedigital circuitry can optionally include other components, which are notdepicted in FIG. 4, including, but not limited to, bipolar transistors;metal-insulator-metal (“MIM”) and metal-oxide-semiconductor (“MOS”)capacitors; diffused, implanted, and polysilicon resistors; and/ordiodes. The digital circuitry can include, but is not limited to, one ormore of a differential amplifier or buffer, an analog-to-digitalconverter, a clock generator, non-volatile memory, and a communicationbus. For example, the digital circuitry can include an on-chip bufferfor storing the respective digital electrical output signals.

In addition to the nMOS and pMOS transistors 210 and 211 of FIG. 4, adoped piezoresistive sensing element 404 and a silicided contact region403 are formed with the same polysilicon gate material used for the nMOStransistor 210 and pMOS transistor 211. In other words, thepiezoresistive sensing element and digital circuitry can be disposed onthe same monolithic substrate. The MEMS force sensor can also include apiezoelectric sensing element 105, which is disposed on the IMD layer102 and underneath solder ball 103. The piezoelectric sensing element105 can be formed after completion of the integrated circuit process.Metal 212 and contact 213 layers can be used to create electricalconnections between nMOS and pMOS transistors 210 and 211,piezoresistive sensing element 404, and piezoelectric sensing element105. Accordingly, the MEMS force sensor includes a piezoresistivesensing element, a piezoelectric sensing element, and digital circuitryall on the same chip.

In addition to the implementations described above, a stressamplification mechanism can be implemented on the substrate of the MEMSforce sensor. For example, as depicted in FIG. 5, the MEMS force sensor500 includes a substrate 101 with a cap 501 bonded to it. The substrate101 and cap 501 can be bonded at one or more points along the surfaceformed by an outer wall 504 of the substrate 101. In other words, thesubstrate 101 and cap 501 can be bonded at a peripheral region of theMEMS force sensor 500. It should be understood that the peripheralregion of the MEMS force sensor 500 is spaced apart from the centerthereof, i.e., the peripheral region is arranged near the outer edge ofthe MEMS force sensor 500. Example MEMS force sensors where a cap andsensor substrate are bonded in peripheral region of the MEMS forcesensor are described in U.S. Pat. No. 9,487,388, issued Nov. 8, 2016 andentitled “Ruggedized MEMS Force Die;” U.S. Pat. No. 9,493,342, issuedNov. 15, 2016 and entitled “Wafer Level MEMS Force Dies;” U.S. PatentApplication Publication No. 2016/0332866 to Brosh et al., filed Jan. 13,2015 and entitled “Miniaturized and ruggedized wafer level mems forcesensors;” and U.S. Patent Application Publication No. 2016/0363490 toCampbell et al., filed Jun. 10, 2016 and entitled “Ruggedized waferlevel mems force sensor with a tolerance trench,” the disclosures ofwhich are incorporated by reference in their entireties.

The cap 501 can optionally be made of glass (e.g., borosilicate glass)or silicon. The substrate 101 can optionally be made of silicon.Optionally, the substrate 101 (and its components such as, for example,the mesa, the outer wall, the flexure(s), etc.) is a single continuouspiece of material, i.e., the substrate is monolithic. It should beunderstood that this disclosure contemplates that the cap 501 and/or thesubstrate 101 can be made from materials other than those provided asexamples. This disclosure contemplates that the cap 501 and thesubstrate 101 can be bonded using techniques known in the art including,but not limited to, silicon fusion bonding, anodic bonding, glass frit,thermo-compression, and eutectic bonding.

In FIG. 5, the cap 501 is made transparent to illustrate the internalfeatures. An inter-metal dielectric layer (IMD) 102 can be fabricated ona surface (e.g., bottom surface) of the substrate 101 to form integratedcircuits. Additionally, a deep trench 502 is formed on the substrate 101and serves as a stress amplification mechanism. The trench 502 can beetched by removing material from the substrate 101. Additionally, thetrench 502 defines the outer wall 504 and mesa 503 of the substrate 101.The base of the trench 502 defines a flexure. The piezoelectric sensingelements can be formed on a surface of the flexure, which facilitatesstress amplification. In FIG. 5, the trench 502 is continuous and has asubstantially square shape. It should be understood that the trench canhave other shapes, sizes, and/or patterns than the trench shown in FIG.5, which is only provided as an example. Optionally, the trench 502 canform a plurality of outer walls and/or a plurality of flexures. Aninternal volume can be sealed between the cap 501 and substrate 101(i.e., sealed cavity). The sealed cavity can be sealed between the cap501 and the substrate 101 when bonded together. In other words, the MEMSforce sensor 500 can have a sealed cavity that defines a volume entirelyenclosed by the cap 501 and the substrate 101. The sealed cavity issealed from the external environment. Example MEMS force sensors havinga cavity (e.g., trench) that defines a flexible sensing element (e.g.,flexure) are described in U.S. Pat. No. 9,487,388, issued Nov. 8, 2016and entitled “Ruggedized MEMS Force Die;” U.S. Pat. No. 9,493,342,issued Nov. 15, 2016 and entitled “Wafer Level MEMS Force Dies;” U.S.Patent Application Publication No. 2016/0332866 to Brosh et al., filedJan. 13, 2015 and entitled “Miniaturized and ruggedized wafer level memsforce sensors;” and U.S. Patent Application Publication No. 2016/0363490to Campbell et al., filed Jun. 10, 2016 and entitled “Ruggedized waferlevel mems force sensor with a tolerance trench,” the disclosures ofwhich are incorporated by reference in their entireties.

A gap (e.g., air gap or narrow gap) can be arranged between the cap 501and the mesa 503, which is arranged in the central region of the MEMSforce sensor 500. The narrow gap serves as a force overload protectionmechanism. The gap can be within the sealed cavity. For example, the gapcan be formed by removing material from the substrate 101.Alternatively, the gap can be formed by etching a portion of the cap501. Alternatively, the gap can be formed by etching a portion of thesubstrate 101 and a portion of the cap 501. The size (e.g., thickness ordepth) of the gap can be determined by the maximum deflection of theflexure, such that the gap between the substrate 101 and the cap 501will close and mechanically stop further deflection before the flexureis broken. The gap provides an overload stop by limiting the amount bywhich the flexure can deflect such that the flexure does notmechanically fail due to the application of excessive force.

Example MEMS force sensors designed to provide overload protection aredescribed in U.S. Pat. No. 9,487,388, issued Nov. 8, 2016 and entitled“Ruggedized MEMS Force Die;” U.S. Pat. No. 9,493,342, issued Nov. 15,2016 and entitled “Wafer Level MEMS Force Dies;” U.S. Patent ApplicationPublication No. 2016/0332866 to Brosh et al., filed Jan. 13, 2015 andentitled “Miniaturized and ruggedized wafer level mems force sensors;”and U.S. Patent Application Publication No. 2016/0363490 to Campbell etal., filed Jun. 10, 2016 and entitled “Ruggedized wafer level mems forcesensor with a tolerance trench,” the disclosures of which areincorporated by reference in their entireties.

This disclosure contemplates that the existence of both piezoresistiveand piezoelectric sensing element types can be utilized to improvesensitivity and resolution of the force sensing device. Piezoelectricsensors are known to be highly sensitive, however their response decaysover time, making them more useful for sensing dynamic forces.Piezoresistive sensors, on the other hand, are more useful for sensingstatic forces. Piezoresistive sensors are known to be less sensitivethan piezoelectric sensing elements. In force sensing applications, itis often necessary to determine the direct current (“DC”) load beingapplied to the MEMS force sensor. In this case a piezoresistive sensingelement, while less sensitive than the piezoelectric sensing element, iswell-suited. In the implementations described herein, the presence ofboth the piezoresistive and piezoelectric sensing elements allows theMEMS force sensor to leverage two signal types and avoid the use ofdead-reckoning algorithms, which become more inaccurate over time.Piezoelectric sensors are highly sensitive, but their operation dependson the generation of charge as stress on the sensing element changes.Piezoelectric sensors are not capable of detecting low frequency or DCsignals, and as such, a static force will appear to decrease over time.To account for this, a filtered piezoresistive signal, which isinherently less sensitive but capable of low frequency and DC signaldetection, can be used to measure the static forces that are acting onthe MEMS force sensor, while a piezoelectric signal, which is moresensitive and capable of higher frequency detection, can be used tomeasure the dynamic forces acting on the MEMS force sensor. In otherwords, piezoresistive and piezoelectric sensors can be used inconjunction to detect both static and dynamic forces acting on the MEMSforce sensor.

As described above, the digital circuitry can be configured to receiveand process both the first analog electrical signal produced by thepiezoresistive sensing element and the second analog electrical signalproduced by the piezoelectric sensing element. The digital circuitry canbe configured to convert the first and second analog electrical signalsinto respective digital output signals, and optionally store the digitaloutput signals in an on-chip buffer. The digital circuitry can beconfigured to use the respective digital output signals in conjunctionin order to improve sensitivity, accuracy, and/or resolution of the MEMSfor sensors.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

1-3. (canceled)
 4. A microelectromechanical (“MEMS”) force sensor,comprising: a sensor die operable to receive an applied force, whereinthe sensor die comprises a top surface and a bottom surface oppositethereto; an electrical terminal at the bottom surface of the sensor die;a piezoresistive sensing element at the bottom surface of the sensor dieadjacent to the electrical terminal, wherein the piezoresistive sensingelement is operable to convert a strain at the bottom surface of thesensor die to a first signal that is proportional to the strain; apiezoelectric sensing element at the bottom surface of the sensor dieand at least partially over the electrical terminal, wherein thepiezoelectric sensing element is operable to convert a change in strainat the bottom surface of the sensor die to a second signal that isproportional to the change in strain; and circuitry at the bottomsurface of the sensor die, wherein the circuitry is operable to: receivethe first signal and convert the first signal to a first output signal;and receive the second signal and convert the second signal to a secondoutput signal.
 5. The MEMS force sensor of claim 4, wherein: the sensordie comprises a substrate and an inter-metal dielectric (IMD) layer overthe substrate; the piezoelectric sensing element is at a surface of theIMD layer; and the piezoresistive sensing element is at surface of thesubstrate.
 6. The MEMS force sensor of claim 4, wherein thepiezoresistive sensing element comprises a doped region of a firstconductivity type formed between at least two doped regions of a secondconductivity type.
 7. The MEMS force sensor of claim 4, wherein thepiezoresistive sensing element comprises one of: a p-type piezoresistivesensing element formed on an n-type substrate; a p-type piezoresistivesensing element formed in an n-type well on a p-type substrate; ann-type piezoresistive sensing element formed on a p-type substrate; oran n-type piezoresistive sensing element formed in a p-type well on ann-type substrate.
 8. The MEMS force sensor of claim 4, wherein theelectrical terminal comprises a solder bump or a copper pillar.
 9. TheMEMS force sensor of claim 4, further comprising a cap attached at thetop surface of the sensor die.
 10. The MEMS force sensor of claim 4,further comprising a sealed cavity between the sensor die and a cap thatis attached at the top surface of the sensor die, the sealed cavitydefining a volume that is enclosed by the cap and the sensor die. 11.The MEMS force sensor of claim 4, further comprising a flexure in thesensor die, the flexure operable to convert the applied force into thestrain at the bottom surface of the sensor die.
 12. The MEMS forcesensor of claim 11, further comprising a gap between the sensor die anda cap that is attached at the top surface of the sensor die, wherein thegap is operable to narrow with application of the applied force suchthat the flexure is unable to deform beyond a breaking point of theflexure.
 13. The MEMS force sensor of claim 4, wherein: the first signalproduced by the piezoresistive sensing element measures static forceapplied to the MEMS force sensor; and the second signal produced by thepiezoelectric sensing element measures dynamic force applied to the MEMSforce sensor.
 14. A method for manufacturing a microelectromechanical(“MEMS”) force sensor, comprising: forming a piezoelectric sensingelement at a first surface of a sensor die; forming a piezoresistivesensing element at the first surface of the sensor die; formingcircuitry at the first surface of the sensor die, the circuitry operableto receive a first signal from the piezoelectric sensing element and asecond signal from the piezoresistive sensing element; and forming anelectrical terminal at the first surface of the sensor die, wherein: theelectrical terminal is operably connected to the circuitry; and thepiezoelectric sensing element is between the electrical terminal and thesensor die.
 15. The method of claim 14, further comprising attaching acap at a second surface of the sensor die.
 16. The method of claim 14,wherein: the sensor die comprises a substrate and an inter-metaldielectric (IMD) layer over the substrate; forming the piezoelectricsensing element at the first surface of the sensor die comprises formingthe piezoelectric sensing element at a surface of the IMD layer; andforming the piezoresistive sensing element at the first surface of thesensor die comprises forming the piezoresistive sensing element at asurface of the substrate.
 17. A microelectromechanical (“MEMS”) switch,comprising: a plurality of electrical terminals at a bottom surface of asensor die; a piezoresistive sensing element at the bottom surface ofthe sensor die adjacent to one or more electrical terminals in theplurality of electrical terminals, wherein the piezoresistive sensingelement is operable to convert a strain at the bottom surface of thesensor die to a first signal that is proportional to the strain; apiezoelectric sensing element at the bottom surface of the sensor diebetween the sensor die and an electrical terminal in the plurality ofelectrical terminals, wherein the piezoelectric sensing element isoperable to convert a change in strain at the bottom surface of thesensor die to a second signal that is proportional to the change instrain; and circuitry at the bottom surface of the sensor die, whereinthe circuitry is operable to: receive the first signal and convert thefirst signal to a first output signal; receive the second signal andconvert the second signal to a second output signal; and provide thefirst output signal and the second output signal to at least oneelectrical terminal in the plurality of electrical terminals.
 18. TheMEMS force sensor of claim 17, wherein: the sensor die comprises asubstrate and an inter-metal dielectric (IMD) layer over the substrate;the piezoelectric sensing element is at a surface of the IMD layer; andthe piezoresistive sensing element is at a surface of the substrate. 19.The MEMS force sensor of claim 17, wherein the piezoresistive sensingelement comprises a doped region of a first conductivity type formedbetween at least two doped regions of a second conductivity type. 20.The MEMS force sensor of claim 17, wherein each electrical terminal inthe plurality of electrical terminals comprises a solder bump or acopper pillar.
 21. The MEMS force sensor of claim 17, further comprisinga cap attached at a top surface of the sensor die.
 22. The MEMS forcesensor of claim 17, further comprising a sealed cavity between thesensor die and a cap that is attached at a top surface of the sensordie, the sealed cavity defining a volume that is enclosed by the cap andthe sensor die.
 23. The MEMS force sensor of claim 17, furthercomprising a flexure in the sensor die, the flexure operable to convertan applied force into the strain at the bottom surface of the sensordie.